1. Field of the Invention
The invention relates to a process for producing molded polyurethane parts or elements by reacting at least one polyol with an organic di- or polyisocyanate in the presence of catalysts and release agents while molding the product. More particularly, the invention relates to a process for producing molded polyurethane parts by the reaction-injection molding (RIM) process.
2. Description of the Prior Art
Molded polyurethane parts are used to a large extent in the automobile industry as car body parts, such as, guard plates, spoilers or fenders. Because of their toughness and abrasion resistance, they are suitable as shoe soles, heels and for many other purposes requiring such properties.
Curing of the polyurethanes takes place simultaneously with the molding process in closed, heated molds. For this purpose, the reaction-injection-molding process or so-called RIM process, has gained particular importance.
In such a process, considerable problem arises because the molded polyurethane parts tend to adhere to the internal walls of the mold. A large number of release agents are known to those skilled in the production of molded parts from reactive composition. Such release agents may be applied on the internal walls of the molds or added as internal release agents to the reaction formulation. These internal release agents belong to several different classes of compounds. As internal release agents, metal soaps, such as, for example, zinc stearate, esters of higher fatty acids, natural or synthetic oils, waxes or silicones are used.
Particularly preferred are those products which do not require any special preparation of the molds, which do not affect the properties of the fully reacted, molded plastic material, and which not or at most minimally affect the surface properties of the molded articles, e.g., impair the paintability. The internal release agents should be evenly distributed in the reaction formulation. However, in order to function as release agents, they must accumulate at the surface of the molded part.
Examples of internal release agents used in the production of molded polyurethane parts by the RIM process, are organosilicon block copolymers which, aside from siloxane blocks, have polyoxyalkylene blocks. In this connection, the structure of the compounds evidently plays a significant role in the release behavior.
For example, German Offenlegungsschrift No. 25 43 638 discloses that compounds of the formula ##STR3## are inferior to compounds having the formula ##STR4## and/or ##STR5## wherein in Formulas II to IV, the subscript x in each case has an average value of 3 to 45, the subscript y in each case has an average value of 8 to 198, Me represents methyl and the (--OR).sub.x radical represents a polyoxyalkylene polymer or a polyoxyalkylene copolymer, R being composed of ethylene radicals or butylene radicals or mixrures of ethylene or butylene radicals with propylene radicals and the amounts of ethylene or butylene radicals in relation to the amount of the propylene radicals being selected so that the ratio of the carbon atoms to the oxygen atoms in the whole block (--OR)-- is 2.0:1 to 2.9:1.
This Offenlegungsschrift also provides test methods for determining and comparing the release properties of various release agents.
More recent work shows that certain functional groups can improve the release properties. Siloxanes with such functional groups are known from U.S. Pat. No. 4,076,695. As functional groups, they contain carboxyl groups which are linked hydrocarbons, which, in turn, are linked to the siloxane frame by means of an Si--C or an SiOC bond. However, these release agents with carboxyl groups do not form stable mixtures with the polyols, which contain amine and, optionally, tin catalysts. Moreover, the gel time of the polyurethane system is lengthened by the addition of these compounds.
Because of their reactivity, these compounds cannot be used as a component of the isocyanate component.
These disadvantages are largely overcome by using as release agents the organosilicon compounds described in U.S. Pat. No. 4,472,341. These siloxanes have ##STR6## units, in which R is a lower alkyl or aryl group. R.sup.1 is a divalent hydrocarbon radical, which may contain oxygen or sulfur atoms. R.sup.2 is a low molecular weight alkoxy or aroxy group or the R.sup.3 O(CHR.sup.4 CH.sub.2 O--).sub.x group, R.sup.3 being a hydrogen or methyl radical and x having a value of 1 to 50. However, their release behavior does not satisfy completely. The same is true for siloxanes with organically bound CONR.sub.2 groups, in which R represents hydrogen radicals.